Why is there no option to convert a mesh to metaball object?
One indirect way to achieve this will be using a grid distributed particle system, with a meta-ball as the particle object.

Here are the important particle system settings for doing this:


And here's the meta version vs. the original:


You can also set the overall resolution via the metaball's resolution property.

Since these are all fairly simple PS properties, it wouldn't be difficult to automate them as a script or addon.